The present invention relates to an automatic level control circuit and, more particularly, to an automatic level control circuit applicable to a demodulator of a multi-level quadrature amplitude modulation (QAM) system and others.
The current trend in the art of microwave band digital communications is toward the use of a multi-level QAM system which enhances effective utilization of the limited frequency band. In such a communication system, an analog-to-digital (AD) converter at a receive terminal compares a demodulated baseband signal with a plurality of predetermined reference levels to thereby transform the baseband signal into two-level parallel digital signals. In this instance, the prerequisite is that the input level of the baseband signal to the AD converter is maintained constant relative to the reference levels despite possible changes in the receive level and those in the gain of a receive amplifier.
The above prerequisite has heretofore been met by installing variable-gain amplifiers one before a QAM detector and the other after the QAM detector and controlling their gains by means of outputs of AD converters.
The problem with the prior art implementation is that, as will also be discussed later in detail, the use of variable-gain amplifiers which are adapted for stabilization of signal level makes the circuit construction complicated and, thereby, brings about the need for extra steps and time for the adjustment of variable-gain amplifiers in the production line.